Two-component polyurethane coatings are in broad use in applications involving plastic or other substrates which are liable to deform on heating or where the coated artifact cannot be heat-treated en bloc in a curing oven as it is true with repair painting of a motor vehicle. The usual two-component polyurethane coating is put to use by mixing a paint component comprising a hydroxyl-containing film-forming resin and an auxiliary component comprising a curing agent, i.e. an organic polyisocyanate compound, each supplied in an independent can. At the coating job site, however, the coating must be adjusted to a viscosity suitable for the coating method to be used, e.g. spray coating. Therefore, after the paint component is mixed with the auxiliary component, the mixture must be diluted with an organic solvent (thinner) not containing active hydrogen. Although such a coating system is generally termed a two-component coating, actually the three components of rein, curing agent and thinner each supplied in an independent can must be blended in suitable proportions at the coating job site.
It would certainly expedite and simplify the preparatory work at the job site a great deal should the required number of cans be reduced from three to two. However, since it is technically impossible to premix the resin with the curing agent and supply the premix in a single can, the only available approach that might be contemplated is to mix the curing agent with the thinner beforehand and supply the mixture in a single can. However, polyisocyanates not only react easily with active hydrogen compounds such as water, alcohols and amines but dimerize, trimerize or polymerize by themselves. Although the neat form of a polyisocyanate is fairly stable, diluting it with a thinner increases the opportunity for such reactions and the resulting dilution tends to undergo clouding (turbidity), precipitation, flocculation, thickening and/or gelation, thus making it virtually unserviceable. Thus, it has been impossible to dissolve the required amount of a polyisocyanate in the total necessary quantity of a thinner beforehand. The inevitable choice was to provide the thinner supplied in an independent can.